The present invention relates generally to metallized plastic film capacitors, and more particularly to structure and method of manufacture of such capacitors utilizing plastic film with increased dielectric constant, and dielectric strength, improved stability, and low dissipation factor compared to metallized film capacitors of the prior art.
A brief treatment of capacitors will be advantageous to an understanding of the invention. In general, a capacitor consists of two conducting metal plates separated by high quality uniform insulating media (dielectric) capable of storing electrical energy at field stress levels approaching the ultimate voltage withstand value, or breakdown voltage value, of the media material. The static capacitance, C, of the device is related to the applied voltage as follows:
C=Q/Vxe2x80x83xe2x80x83(1)
where the capacitance of the capacitor measured in units of farads (F) is equal to the quantity of charge Q in coulombs which is stored on the positively charged metallic plate of the capacitor, divided by the total potential difference V in volts across the plates. Geometrically:
C=eeoA/txe2x80x83xe2x80x83(2)
where A is the area of each plate, t is the thickness of the insulating media layer of dielectric constant e, and eo is the dielectric constant of free space. The energy, E, in joules (J) stored in the capacitor at a potential difference V across the plates is:
xe2x80x83E=xc2xdC(V)2xe2x80x83xe2x80x83(3)
The energy stored in a charged capacitor can be continuously increased in proportion to the increase of the voltage, up to high values of V, limited only by the electrical breakdown of the dielectric. It would appear, then, that the most significant increases in the energy density of a capacitor may be made either by increasing the dielectric constant e of the insulating media, or by increasing the applied voltage (field stress) V, or both. The solution, however, is not that simple. In some cases, an increase in the dielectric constant will lead to an increase in dielectric losses, leading to thermal management problems and, worst case, to thermal failure of the capacitor. And an increase in the applied field stress can lead to low reliability and early failure from several possible failure mechanisms which include electromechanical, thermal, chemical and partial discharge mechanisms, to name a few.
A typical conventional metallized film capacitor is the wound capacitor. Dielectric material used in this and other film-type capacitor designs include Kraft paper and various polymer films such as polyester, polypropylene and polycarbonate. The capacitor is formed by sandwiching the dielectric film between metal electrodes (the capacitor plates, which may, for example, be discrete foils or vapor deposited metal film). Use of metallized film reduces capacitor size, but at the expense of peak and average power capability. Connections are made to the electrodes either by extending one entire edge of an electrode out one end of the winding and soldering, arc, flame-spraying or silver-epoxying connections at each end, or by inserting wires or flattened tabs into the winding in contact with each electrode. Examples of wound capacitors are disclosed in U.S. Pat. Nos. 4,719,539 and 4,685,026 to Lavene, U.S. Pat. No. 5,384,684 to Sugisawa, and U.S. Pat. No. 5,406,446 to Peters.
Plastic film capacitors have been the capacitor of choice for many power electronics and pulse power applications because of their inherent low losses, excellent high frequency response, low dissipation factor (DF), low equivalent series resistance (ESR) and high voltage capabilities. Film capacitors have no capacitance coefficient with applied voltage, and metallic migration or leaching does not occur as observed in ceramic capacitors. The film molecule is stable over long term use and is not prone to dielectric dissipation factor degradation or metallic shorting mechanism. Table 1 shows typical properties of some of the common film dielectrics in use today.
Polyethylene terephthalate (polyester or PET) offers a reasonable dielectric constant, has a higher operating temperature of 125xc2x0 C., and is available in film thickness of less than one micron (xcexcm). However, PET has relatively higher DF with increasing temperature and frequency. For high repetition rate, PET is unsuitable for high pulse power applications.
Polypropylene (PP) has inherently low losses, excellent frequency response and very low DF and ESR with temperature and frequency. In fact, the material possesses a negative temperature coefficient of dissipation factor. The PP chain molecules do not possess polar groups, which are oriented under the effect of electric fields. It is this phenomenon which gives rise to the above beneficial properties. It has the highest breakdown voltage of any capacitor film material. Its only negative may be its maximum operating temperature of 105xc2x0 C.
Devices made with polyethylene napthalate (PEN), polycarbonate (PC) and polyphenylene sulfide (PPS) dielectrics also have extremely stable characteristics over extremes of voltage, temperature and frequency. Although the intrinsic breakdown voltage for most of these film dielectrics is quite high, in full wound capacitors these dielectrics are usually derated by a factor of 6 to 8 for improved cycle life and reliability.
The polar polymer polyvinylidene fluoride (PVDF) exhibits a large dielectric constant (xcx9c12) and demonstrates excellent piezoelectric and pyroelectric properties. PVDF is a partially crystalline linear polymer with a carbon backbone in which each monomer {CH2xe2x80x94CF2xe2x80x94} unit has two dipole moments, one associated with CF2 and the other with CH2. In the crystalline phase, PVDF exhibits a variety of molecular conformations and crystal structures depending on the method of preparation. The extruded or cast material usually contains 40 and 60% crystalline material in one or both of the principal crystalline phases, alpha and beta. The alpha phase predominates in material cast from the melt. This phase is converted to the beta phase by mechanical deformation of the material at temperatures less than 100xc2x0 C. In commercial production, PVDF film is extruded and mechanically stretched both parallel and perpendicular to the direction of extrusion, as are most of the capacitor grade film dielectrics. This causes a preferred orientation of the polymer chains in the plane of the film and also converts a large percentage of crystallites to beta form. It is this bi-axially oriented film material which, after polarization, forms the basis of piezoelectric and pyroelectric devices. Unfortunately, the highly crystalline structure also results in some weakness in the physical strength of the film. This causes major problems during the manufacture of very thin films in gauges of less than 5xcexc.
Metallized film capacitors offer the highest volumetric and gravimetric energy densities and reliability of all designs of film capacitors and offer higher pulse power capabilities than foil and other designs. Early film capacitors for high pulse power applications were of dielectric film/foil construction, impregnated with dielectric fluid that filled any voids between layers, and typically had energy densities of less than one J/cc. More recent improvements to these pulse power devices include use of metallized polymer films as the dielectric, providing higher energy density and greater reliability. Fluid impregnated film capacitors have a very narrow operating temperature range while the metallized version can operate up to 100xc2x0 C. with the exception of PPS and PTFE, which can reach an operating temperature of 200xc2x0 C. Plastic film capacitors can be tailored for very high voltages simply by adjusting the film or dielectric thickness in the capacitor.
The thin metallization layer on a metallized film capacitor is capable of vaporizing away if a short circuit or a weak location occurs somewhere in the dielectric. This phenomenon is known as self-healing or clearing. Clearing should only result in metal oxide insulator formation. Weak locations are usually caused by localized thin spots, or xe2x80x9cbubbles,xe2x80x9d during film manufacture, or due to an impurity within or on the surface of the film, or due to a conductive xe2x80x9ctrackxe2x80x9d within the dielectric layer. These are adverse features that are common to all polymer film dielectric materials.
For high pulse power applications, metallized film capacitor dielectrics typically have been either PP or PVDF, the former being the preferred polymer for high repetition rate applications because of its extremely low DF, while the latter is the preferred polymer for low repetition rate applications because of its high dielectric constant (12) compared to films such as PP (2.5). To the knowledge of the applicant herein, the highest energy density attained to date using PVDF film material in high pulse power devices is about 2.4 J/cc. PVDF appeared to have promise in various applications, but suffers from non-linearity of capacitance with voltage, very poor insulation resistance, poor clearing ability, high leakage current, relatively low voltage breakdown, poor mechanical properties, and unavailability in thin gauge and uniform thickness. PVDF suffers from poorer performance at elevated temperatures, with application of voltage producing a larger number of clearing sites than at room temperature. In addition, PVDF has relatively high cost. While PP has a poor dielectric constant that limits its overall energy density, it is otherwise an excellent dielectric insulator.
In addition to high pulse power applications, a large market exists for a highly energy dense capacitor in the medical sector, such as in implantable defibrillators for treatment of ventricular fibrillation and other cardiac dysrhythmias. The energy density of currently available electrolytic capacitors for a 30 Joule (15 cc) defibrillator is about 2 J/cc. A 30 J high voltage film capacitor with energy density of at least 5 J/cc would occupy only about 40% of presently used capacitor volume, have no reform or outgassing, low ESR, and thus could allow use of a smaller battery to charge the capacitor, so as to provide a significant reduction in overall defibrillator size. The device is implanted in the patient""s pectoral region, but is typically considerably larger than implantable pacemakers. The batteries and capacitors occupy approximately 70% of the total space, so reduction in size of those components would lead to significant savings in device size.
Specification of capacitors for use in implantable defibrillators presents a unique challenge because of a need for high power and energy density in a small package. Physiological uniqueness is also present because a relatively high voltage is required to achieve successful defibrillation, and the energy must be delivered in a time frame measured in milliseconds (ms). While recent studies have indicated that defibrillation in humans may be possible with relatively lower voltages and energies than have been thought to be required in the past, the high voltages presently used are essential to provide the highest confidence level of achieving a successful defibrillation.
Current major capacitor requirements for an implantable defibrillator include:
700 to 800 V/100 to 150 mF
Energy delivery of 20 to 40 J in 10-20 ms pulse
Longevity of greater than 5 years
Energy density greater than 5 J/cc
Capacitor sits in the uncharged state
Nominal leakage current of less than 100 microamperes (xcexcA)
Operation in an isothermal (37xc2x0 C.) environment
In practice, aluminum electrolytic photoflash capacitors typically are used, which exhibit an energy density of about 2 J/cc at operating voltages of about 375 V under these conditions. This capacitor maximizes the surface area of the electrode and can be fabricated in thin film configuration to optimize the energy density. But the system holds little promise of volume and weight reduction. The system is also prone to possible energy reversal, which can lead to deformation of the anodic oxide film, resulting in decreased energy storage efficiency. Despite their high capacitance and certain other advantages, the aluminum electrolytic capacitors suffer from such disadvantages as reforming periodically (with attendant consumption of valuable battery energy), physical size, high dissipation factor, low voltage (two required per defibrillator), hydrogen liberation during charge and storage (either the capacitor or the integrated circuit in the defibrillator must be hermetically sealed for protection), thermal runaway at high sustained voltage, short shelf life, and fluid outgassing.
Medical applications of capacitors beyond the implantable defibrillator include external defibrillators, microstimulators, and cochlear implants.
An appropriate solid state film capacitor could circumvent the problems encountered with the electrolytic capacitor and provide various benefits, such as no reform (with consequent conservation of battery energy), lighter weight, high energy density (possibly greater than 5 J/cc), monolithic (i.e., only one component required to provide 700 V), no outgassing, greater reliability and safety, wide operating temperature range, flexible form factor, and relatively much lower cost. Recent innovations in film material processing has led to incremental improvements of 20 to 30% in energy density and other properties of the film.
One area of improvement is described, for example, in U.S. Pat. No. 5,614,111 to Lavene, and in a publication by G. J. Walters, in 17th CARTS, Mar. 24-27, 1997, where the metallization is made as thin as possiblexe2x80x94from 5 to 300 xcexa9/sq as opposed to the 1 to 4 xcexa9/sq of the typical industry metallization thicknessxe2x80x94to increase the film""s dielectric strength (voltage breakdown). Dielectric breakdown for PEN and PET using this metallization process is higher by 20 to 100%, but this is insufficiently significant to provide the energy density of 5 J/cc required by the implantable defibrillator.
Another area of improvement is in coating the polymer film dielectric with a thin film of acrylate material (e.g., 0.3 to 1 micron) before metallizing to increase breakdown voltage and energy density, and to improve clearing ability of the polymer film. This is described, for example, in PCT application publication No. WO 97/37844 to Yializis. Initial data for PET film suggests an improvement in breakdown voltage by about 10-20% on thin films (2 microns) and 30-50% on ultra-thin films (less than 1.5 microns). The improvement for thick films (greater than 6 microns) is negligible. For thin film PET and PP, the acrylate coating improves energy density by about 20%. The mechanism for these improvements is unclear. One hypothesis is that the acrylate provides extra oxygen on the polymer dielectric for more efficient burning and attendant improved clearing. But PP has no oxygen in its structure, and yet exhibits the best clearing of all capacitor film material presently available, even without the acrylate coating. Another hypothesis is that the presence of acrylate coating allows film processing into capacitors with less damage because of hardness of the coating, which prevents pinholes and other mechanical degradation. Another hypothesis is that the acrylate has a slightly higher dielectric constant than PP or PET, and that dominates the energy term in the capacitor.
It has been generalized in the film capacitor industry that polymer film material with oxygen in its structure clears better than those without oxygen. In poorer clearing materials, such as those with oxygen deficiency, carbon accumulates at the clearing site, resulting in catastrophic failure from conductive shorts. This may be true to some extent, but the applicant herein submits that other contributing phenomena may determine whether or not a material clears well. This is based on a number of other observations, e.g., polyester has plentiful oxygen in its structure but does not clear as well as polypropylene which has no oxygen in its molecular structure; polyester is relatively polar while polypropylene is non-polar; and polar molecules are more leaky than non-polar molecules. It has also been observed that major failures occur at higher voltages than at lower voltages.
Clearing occurs at a point in the polymer film where the weak spot reaches a limiting voltage lower than the intrinsic voltage breakdown of the polymer. Since electrical stresses are involved in this phenomena, it is reasonable to assume that orientation changes occurring within the polymer as a result of the applied stress are also major contributing factors in the breakdown. The applicant herein is of the opinion that an important factor that may contribute to the poor clearing ability of dielectric polymer films is the orientation phenomenon that occur as a result of the electrical stress.
Polyester, despite having considerable oxygen in its molecular structure undergoes some finite levels of polarization and when clearing occurs, the molecular orientation phenomenon results in a finite clearing residence time. The higher the voltage level, the greater the orientation of the molecules and the greater the residence time during clearing of any weak locations. This allows more carbon residues to accumulate at the weak location in the case of higher voltage levels; hence, resulting in a major failure. With PVDF, the molecule is highly polar, causing increased orientation even at lower voltage levels. It is because of this phenomenon that PVDF is electrically more active than the other polymers and this is reflected in its poor DF and breakdown properties. Likewise, polyester is also somewhat polar and has correspondingly lower DF. However, with polypropylene, the fact that no polarization occurs suggests that this molecule clears instantly the moment breakdown voltage of the weak spot is reached. Since the residence time for clearing is anticipated to be very short, relatively little carbon accumulation is expected, and hence major failure as a result of this phenomenon is minimized. The polypropylene molecule has the best breakdown voltage, best clearing ability and lowest DF of any of the film dielectrics.
Yet another application for improved film capacitors is for surface mount chip capacitors. This has led to the emergence of several higher temperature polymers such as PPS, PPO, PEN and PEEK. These materials are slow in penetrating the capacitor film market because other key properties such as clearability and insulation resistance are often inferior to the lower temperature films. The need for even higher operating temperature has fueled development efforts to produce films that will withstand temperatures greater than 260xc2x0 C.
It is a principal objective of the present invention to overcome many of the problems of traditional film capacitors and aluminum electrolytic capacitors, and to offer an alternative power source with significantly improved performance capabilities over these prior art devices.
The primary object of the present invention to provide a polymer film material with the following properties, vis-a-vis, improved dielectric constant, improved breakdown voltage, improved dissipation factor, improved clearability, lower leakage (and hence higher insulation resistance), and higher operating temperature capabilities, and which can be wound in a capacitor to yield an energy density exceeding 5 Joules per cubic centimeter (J/cc). This is achieved by providing a hybrid polymer film material which is a unique copolymer solid-solution blend of a higher dielectric constant material, or a higher temperature resistant material and at least one non-polar dielectric material component. The invention is not limited to two types of dielectric blends but can be extended to three or more resin blends to tailor the appropriate properties.
In a novel capacitor design of the applicant herein described in U.S. patent application Ser. No. 09/065,131, the film capacitor utilizes a wound bi-layer of PVDF and PP (i.e., four film layers in total are wound together in that instance, instead of the traditional method of winding two single layers of similar polymer films with a metallization layer between the two dielectrics) that combines the excellent insulator properties of PP with the excellent dielectric constant of PVDF, to obtain a performance improvement of more than 50% over a single PVDF layer. The improvement is enhanced by increasing the breakdown strength of PVDF/PP combined material when compared to PVDF alone. Nevertheless, variations in thickness and film quality of the bi-layers, intrusion of air between the bi-layers during winding (e for air is 1, with breakdown voltage of only 3 volts per micron compared to several hundred volts per micron for either PVDF or PP and significantly higher dielectric constant), differing thermal electrical properties of the PP and PVDF films in the bi-layers, physical adhesion compatibility issues of the bi-layers, as well as the instability of PVDF homopolymer film, and lack of optimum insulation properties, may combine to limit the long-term reliability of the prior design. By winding four layers together, a greater probability exists of introducing poor uneven windings as a result of the variation in the film thickness. In addition, the prior design does not alter the chemical or electrical properties of the homopolymers, i.e., PVDF is still PVDF, with poor individual electrical properties, and PP is still PP, with poor dielectric constant. In addition, the polar form of PVDF still remains as long as it is in a homopolymer form and just a physical lamination of PVDF and PP.
The present invention overcomes many of the problems of the prior art, and of the aforementioned prior design, through the use of two single layers of new designs of hybrid copolymer film materials in the construction of metallized film capacitors similar to the construction of traditional film capacitors. The invention reduces the introduction of air intrusion during capacitor construction, as observed in the construction of four film layers, since only two film layers are usedxe2x80x94hence, increasing the probability of obtaining high performance of the improved film material rather than manufacturing flaws. It also provides a more precise method of tailoring polymer blends with specific film properties for specific applications that cannot be achieved from homopolymer film whether the latter are single layer or combined as bi-layer, tri-layer, etc., films. Since the new material is a copolymer solid solution as opposed to a homopolymer bi-layer, the two copolymer layers used in capacitor fabrication do not present the same thermal/electrical issues as are described above for PVDF/PP bi-layer winding.
A number of new designs of film material are possible with this invention simply by choosing the appropriate initial materials and tailoring the blends for the intended application. For example, in the design and construction of a capacitor for an implantable defibrillator that would require about 5 or more Joules per cc, the selection of a material with a very high dielectric constant, good clearing ability and breakdown voltages would be necessary and desirable. A representative example of two polymers that could be blended into a copolymer to meet these needs is PVDF and PP, but the composition chosen and the specific percentages of the components will depend on the specific requirements of the film capacitor in each particular instance. In this example, by blending the two resins and manufacturing thin films of the copolymer, a material can be obtained in which the highly polar activity of PVDF is reduced and stabilized through the formation of the copolymer. The reduction in the polarization activity is further reflected in the improvements in the electrical properties of the film, including an increase in the breakdown voltage and insulation resistance of the copolymer compared to PVDF alone, and the ability to be manufactured in thin film with increased physical strength through the reduction in the crystallinity of the copolymer. The result is a material with enhanced energy density and electrical stability over PVDF homopolymer alone.
Such design principles are readily extended to other polymeric insulators such as polyester, or polycarbonate or any other dielectrics to tailor special properties desired for a particular application. Very thin metallized film capacitors designed in accordance with this hybrid copolymer techniquexe2x80x94which is not limited to a blend of only two polymers but may extend to three or morexe2x80x94enable achieving a device with stable dielectric constant and, hence, stable capacitance with voltage, improved insulation resistance and clearing or self-healing ability, lower leakage currents, and higher voltage breakdowns (compared, for example, with homopolymer PVDF), with the potential for unprecedented energy density from a bulk capacitor system. The markedly higher performance values (energy density, reliability, weight) is anticipated to be matched by markedly lower cost per unit of performance when volume manufacturing is employed. The methodology of the invention is well suited for the production of the implantable defibrillator, for example, and in many other high pulse power applications where energy density is afforded a premium.
Another example of the use of these techniques to achieve new designs is in the development of capacitors for high temperature applications. For example, as indicated in Table 1 above, PPS and PTFE have operating temperature ranges up to about 200xc2x0 C. Although PTFE can be used at slightly higher temperature, at higher temperatures the electrical properties of the dielectric become very poor. By combining blends of PVDF and PP to either PPS or PTFE in different proportions, the energy density of the capacitor can be increased, and the breakdown strength and other electrical properties can be stabilized for use of the material at temperatures in excess of the indicated range of the homopolymer. This can also be applied to moderate temperature range polymer films such as PET, PEN or PC, to enhance the electrical properties simply by stabilizing with one or more hybrid copolymer components consisting of at least one non-polar group.
According to one preferred embodiment and method of the present invention, homopolymers of high purity (i.e., greater than 99%, preferably greater than 99.9%, pure) PVDF and PP resins are blended and co-extruded (e.g., by twin screw blending) with homogenization to form a melt-cast hybrid copolymer dielectric film. The process results in a thick film, e.g., having a thickness at the lower (thinner) end of a range from about 100 to 200 microns (micrometers, xcexcm), which requires stretching to make it thinner. The concentration of PVDF in the polymeric hybrid is 1 hundredth to 99 hundredths parts of PVDF, with the balance PP (i.e., 99 hundredths to 1 hundredth part PP).
It will be understood, however, that a specific constituent or concentration of either (or any) constituent in a polymeric blend according to the invention is adjusted so as to tailor the properties of polymeric dielectrics for different applications. For an implantable defibrillator, for example, high energy density is required as well as good DF and breakdown voltages. In that instance (returning to the discussion of the exemplary preferred embodiment commenced above), it is necessary to maximize the PVDF content for the energy requirement and to balance it accordingly with PP to stabilize the poor electrical properties of PVDF. However, to obtain some desired property(ies) other than simply energy density, it may be necessary to reduce the concentration of PVDF to as low as 1 part PVDF to 99 parts PP. It is anticipated that an optimum composition for high performance is in the ratio of at least 1:1, but it is not intended that the specific ratio or concentration of the various component resins in the blend shall constitute a limitation on the breadth or scope of the invention.
For purposes of thinning, the melt-cast film is then bi-axially oriented via machine direction orientation (MDO) and transverse direction orientation (TDO) stretching, to a final thickness in a range from about 0.5 xcexcm to 25 xcexcm. Such processing is, in and of itself, completely conventional in the art of polymer film extrusion for capacitor manufacture or food packaging (e.g., in the latter case, production of Saran Wrap(copyright), the common household plastic film used to cover food materials). The base hybrid film is then coated to thickness in a range from 0.1 xcexcm to 2.0 xcexcm, for example, with a polymeric material, such as an acrylate, in which the coating may be applied by doctor blading an acrylate solution or by atomization spray, followed in either case by radiation curing. The coating should have properties of good dielectric constant (e.g., 2.5 to 16) and excellent stability (i.e., improved DF and breakdown voltage, etc., relative to PVDF), and is effective to seal any defects including pinholes as well as to harden the surface of the film to some degree. The coated hybrid film is then metallized with a layer of an appropriate metal, such as aluminum, to a thickness typically in a range from 50 xc3x85 to 500 xc3x85 (Angstroms) by a conventional metallization technique, to provide one electrode or plate of a capacitor, with resistance ranging from 0.1 ohm per square (xcexa9/sq) to 1000 xcexa9/sq. Finally, the film is tightly wound with another correspondingly formed coated hybrid metallized film to the required capacitance, and in some cases impregnated with high dielectric constant fluid, and hermetically sealed.
It will be seen, then, that the present invention represents a distinct improvement over the invention disclosed in the aforementioned ""131 application wherein homopolymers of PVDF/PP (4 layers) are employed, in contrast to use, according to the present invention, of copolymer blends, which may include coating by acrylate. Two single metallized layers of copolymer provides a much more simple implementation of a capacitor than using multiple bi-, tri-, or greater numbers of layers of many films.
The hybrid copolymer film material of the invention offers high dielectric constant, improved stability, improved dissipation factor, improved clearing ability, and high breakdown voltage. Although this film is particularly well suited for film capacitor applications, it is also useful in electrical cables, magnetic tapes, optical films for security and other purposes, piezoelectric sensors, and food packaging, to name a few other applications.
Therefore, another principal aim of the present invention is to provide an improvement over the invention of the aforementioned ""131 application by means of a copolymer solid-solution blend of PVDF and PP to form a hybrid copolymer material which chemically stabilizes the high activity of PVDF and provides improved electrical properties over PVDF alone, in a material with enhanced energy density and electrical stability.
Although a two-polymer blend represents a preferred embodiment, with at least one non-polar component in the blend, the present invention is readily extended to a three or more polymer blend which tailors the specific properties desired for the final polymer. The preceding brief description of copolymer blends comprising PVDF and PP is provided merely for the sake of simplicity and clarity of exemplary embodiments of the invention for high energy density capacitors for use in implantable defibrillator and other high pulse power applications. It will be understood by those skilled in the art that other polymer hybrid blends may be fabricated from a combination of two or more of PVDF, PP, PEN, PPS, PC, PET, PTFE, or other polymeric materials possessing high insulation resistance such as those based on acrylates or polyethylene oxide (PEO) or polypropylene oxide (PPO), for these and other applications. For example, design and fabrication of a hybrid copolymer blend film material for improved or higher temperature applications and with improved electrical properties, may be achieved using a tertiary copolymer blend of PPS, PVDF and PP.
The hybrid copolymer of the invention enables the design of very thin metallized film capacitors with stable dielectric constant and stable capacitance with voltage, as well as improved insulation resistance and clearing or self-healing ability, lower leakage currents and higher voltage breakdown compared, for example, with homopolymer PVDF, with the potential for energy density greater than 8 J/cc from a bulk capacitor system. This represents a more than three-fold increase over state of the art PVDF film capacitors, and a more than six-fold increase over other polymer films, in energy density. Further, the cost of the hybrid capacitor of the present invention could be about 50% lower than existing film capacitors on a per unit energy basis, with the economies of scale of volume manufacturing.
At about 600 volts per micron and a dielectric constant of about 12, the intrinsic energy density of PVDF is about 19 J/cc, and the intrinsic energy density of PP is about 3.5 J/cc. In practice, PP capacitors have achieved 1 to 1.5 J/cc, representing about 30% to 40 % of their intrinsic value; whereas PVDF capacitors have attained only about 12% of their intrinsic value. The poor dielectric properties of PVDF (except for dielectric constant) appear to be responsible for its low practical yields. Use of acrylate coating or lighter metallization may improve PP""s properties, but not necessarily lead to higher levels of energy density. Further, such modifications for PVDF may only lead to about a 20% increase in its energy density through improved breakdown, with no effect on its other poor properties such as high DF and poor insulation resistance, reliability and mechanical properties, among others, and may still not render the material suitable for applications requiring high repetition rates.
By combining the desirable properties of these two materials in a copolymer design, a hybrid polymeric dielectric is achieved with considerable beneficial results. Their relatively close melting pointsxe2x80x94PVDF at 171xc2x0 C., and PP at 189xc2x0 C.xe2x80x94ensures good melt blending of the two polymers, and similar rates of cooling without polymer segregation. The applicant herein is not aware of any other film dielectric material that would serve to enhance energy density by a two- to three-fold increase over state of the art PVDF film capacitors.
A further aim of the invention is to enhance the properties of the hybrid film by coating it with a material, such as acrylate, that has a good dielectric constant and high stability. This improves the base film by sealing defects and pinholes, and further, by hardening the surface to some degree. Other materials that would serve as such a coating include, without limitation, PEO, PET, PPS, PC, PTFE and PEN film.
Still another objective of the invention is to improve the voltage breakdown and clearing ability of the hybrid film by use of lighter metallization processes.
A further objective is to enhance the performance of the hybrid film capacitor material, especially for high energy and high pulse power applications, by impregnating the wound film materials with a high dielectric fluid in a hermetic design. Aromatic compounds such as butyl phenyl sulfone, isopropyl phenyl sulfone, and others, have very high dielectric constantsxe2x80x94exceeding 30xe2x80x94and wetting abilities, better than standard castor oil and trecresyl phosphate, for use in high energy and high power applications. Representative materials are described, for example, in U.S. Pat. No. 4,912,596 to Kron.
Yet another objective of the present invention is to provide such hybrid film materials constituting blends of PVDF, PP, PEN, PET, PPS, PTFE, PC, for example, and various copolymers of such materials, by manufacturing methods such as bi-axial extrusion, or blown bubble process, or melt cast or solvent casting techniques, or vapor deposition onto a substrate.
Still another aim of the invention is to provide a thin coat of a material of high dielectric constant and relatively low electrical properties, such as PVDF, onto a capacitor grade polymer film of lower dielectric constant but higher electrical properties, such as PP, PET, PEN, PPS, PC or PTFE, or copolymers or hybrid polymers formed from such blends. The coating material thickness ranges from 0.1 micron to 25 microns, and the coated substrate thickness ranges from 0.5 micron to 25 microns. The coating can be solvent cast directly onto the polymer substrate, or vapor deposited in an atomized manner, or melt cast directly onto another melt cast substrate, or heat laminated. The coating can be applied to either MDO or TDO substrate polymer film. If an MDO substrate is used, the coated film could be stretched subsequently in the TDO direction, to achieve bi-axial direction orientation for the coating.